In many cases it is necessary to remove H.sub.2 S from gas mixtures, e.g., in order to render these gas mixtures suitable for catalytic conversions using sulphur-sensitive catalysts, or in order to reduce environmental pollution, if before or after combustion the gas mixtures are to be discharged to the atmosphere. Examples of gas mixtures from which H.sub.2 S generally has to be removed are gases obtained by complete or partial combustion or gasification of oil and coal, refinery gases, town gas, natural gas, coke-oven gas, water gas, propane and propylene.
In many cases the H.sub.2 S will be removed from the gas mixtures using liquid absorbents. In a large number of cases the gas mixtures to be purified will also contain CO.sub.2, at least some of which will be absorbed in the liquid absorbent with the H.sub.2 S. The H.sub.2 S and CO.sub.2 will be removed from the said gas mixtures at the pressure of the gas mixture concerned, i.e., in many cases at elevated pressure. By heating the loaded liquid absorbent (liquid absorbent is hereinafter also referred to as absorbent), for example, with steam, an acid gas stream is obtained containing substantially H.sub.2 S and CO.sub.2 in the ratio in which these substances were present in the loaded absorbent.
This H.sub.2 S and CO.sub.2 -containing acid gas cannot be discharged to the atmosphere before at least most of the H.sub.2 S has been removed. The H.sub.2 S is suitably removed from this gas by converting the H.sub.2 S into elemental sulphur.
The gas mixture is preferably supplied to a unit in which the hydrogen sulphide is converted into elemental sulphur, such as the Claus process. In this unit, a proportion of the H.sub.2 S is oxidized to SO.sub.2, which in reaction with the remaining H.sub.2 S, possible under the effect of a suitable catalyst, forms sulphur and water. This conversion of H.sub.2 S into elemental sulphur proceeds more readily when the H.sub.2 S concentration in the acid gas is above a certain minimum value. Thus, special measures become necessary if the H.sub.2 S concentration is lower than 15%. An H.sub.2 S concentration of less than 2% renders the use of the Claus process, even with special measures, such as direct oxidation or fuel gas injection, no longer attractive.
The object of the invention is to provide a method for automatically increasing the concentration of hydrogen sulphide in an acid gas which method is reliable, simple to perform, relatively less expensive and moreover flexible.
To that end, the method according to the invention is characterized in that a value is determined for the magnitude of the gas stream passing to the absorber; in that, if this value is smaller than a predetermined value, a proportion of the liberated or acid gas is passed to a second selective absorber, in which it is contacted with a proportion of the regenerated absorbent originating from the regenerator, which absorbent, when loaded, is then supplied to the regenerator, and that if the said value again rises above a second predetermined value, the second absorber is shut down.
The magnitude of the gas stream determines, among other things, the degree in which the absorbent also absorbs carbon dioxide, in such a way that if there is a small gas stream, much more CO.sub.2 is absorbed than if this stream has a high value. The gas liberated from the absorbent after regeneration contains not only hydrogen sulphide but also carbon dioxide. Now if, for example, the hydrogen sulphide represents only 2% of the total quantity of gas, then it is not desired to pass this gas stream to the Claus process.
According to the invention a proportion of the liberated gas stream is passed to a second absorber, in which the conditions are so selected that preferably H.sub.2 S is absorbed in the absorbent, or, in other words, that H.sub.2 S is selectively removed from the gas stream. The resultant loaded absorbent is also passed from the second absorber to the regenerator, where the gas is liberated from the absorbent, as is the gas originating from the loaded absorbent of the first absorber. It will now be clear that the combined gas stream originating from the regenerator has a higher hydrogen sulphide content than the gas leaving the regenerator before the second absorber was actuated. A new equilibrium adjusts itself at a desired H.sub.2 S concentration. If, subsequently, the gas stream to the first absorber begins to increase again, at a certain moment the situation will be reached that the measured value exceeds a second predetermined value whereupon the gas stream to the second absorber is stopped. Preferably this second predetermined value is selected larger than the first predetermined value in order to be sure that this automatically proceeding process continues to be controlled in a stable manner.
At the moment that it is established that the magnitude of the incoming gas stream has become too small, the H.sub.2 S concentration of the discharged gas is still sufficient, to the extent of course that the quantity of H.sub.2 S in the supplied gas remains reasonably constant; the effect of the reduced supply will only become felt at the outlet of the plant after some time. Hence the control of the gas stream to the second absorber must only be initiated some time after the reduced supply has been observed. In order to have the second absorber in operation at the moment when the gas begins to flow to this absorber, the absorbent circulation is generally started immediately the magnitude of the quantity of the gas flowing to the first absorber has fallen below a certain desired value; a corresponding signal is subsequently passed via a delay device to open the line through which the gas is passed to the second absorber.
If the hydrogen sulphide concentration in the supply to the first absorber is not substantially constant, it is preferred when comparing the value of the quantity with the predetermined value also to take into consideration the concentration of hydrogen sulphide in the liberated gas. In this way it is possible to keep the quality, i.e. the H.sub.2 S concentration of the liberated gas, within certain limits.